ROCKS

The Earth is covered in a layer of solid rock called the
crust. Rocks are either SEDIMENTARY, IGNEOUS,
or METAMORPHIC. Almost all rocks made of minerals, but different
rocks contain different mixtures of minerals. Granite, for example, consists of
quartz, feldspar, and mica. A rock can be identified by its overall colour, the
minerals it contains, the size of the mineral grains, and its texture (mixture
of grain sizes).

BEDROCK

The solid rock that makes up the Earth’s crust is called
bedrock. It can be seen on coasts
and in mountains, where it is being worn away by erosion. Erosion breaks the
bedrock into small pieces, forming soil and sediments (such as mud, sand, and
gravel), which cover up the bedrock in most places. The sediments may later
turn into sedimentary rocks.

SEDIMENTARY ROCKS

Sedimentary rocks are made of particles of sediments such
as sand and clay, or the skeletons and shells of sea creatures. When layers of
loose sediment are buried and pressed down under more layers, the particles
slowly cement together and lithify (form rock). Chemical sedimentary rocks,
such as flint, form when minerals dissolved by water are deposited
again.

Flint is hard and breaks into sharp pieces. It forms from silica
in seabed sediments and grows into nodules with an irregular shape. It is often
found as bands within chalk.

Chalk is soft, white, and fine-grained limestone. It is made from
the remains of microscopic sea creatures, which were deposited at the bottom of
ancient seas.

Conglomerate is made up of rounded pebbles embedded fine-grained
rock. It is formed when shingle is buried by other sediments.

Clay is a very fine-grained sedimentary rock. It is soft and
crumbly when dry, and sticky when wet. Buried clay gradually turns to claystone
and shale.

Sandstone is a medium-grained sedimentary rock made from sand
grains. It is formed when sandy beaches or river beds are buried by other
sediments.

IGNEOUS ROCKS

Igneous rocks are created when magma (molten rock under
the Earth’s crust) cools and becomes solid. Magma loses heat when it
moves upwards at weak spots, such as cracks, in the crust. Extrusive igneous
rocks form when magma reaches the surface and cools quickly. Fast cooling
produces fine-grained rocks. Intrusive igneous rocks form when magma cools
slowly underground. This allows the minerals to grow into coarse
grains.

The Organ Pipes at Twyfelfontein in Namibia are made of basalt.
This extrusive rock formation occurs when lava (volcanic magma) erupts and
solidifies. The basalt cracks into flat-sided columns because it shrinks as it
cools. Basalt is the Earth’s most common igneous rock.

Dark, shiny obsidian is also known as volcanic glass. It is formed
when volcanic lava cools so quickly that crystal grains do not have time to
form. Prehistoric people used obsidian to make sharp tools.

Granite’s colour varies with the minerals it contains. This
pink granite shows grains of pink feldspar, white quartz, and black mica. It is
formed by slow cooling of molten rock deep in the Earth.

Porphyrite (also called microdiorite) is a grey or dark grey
intrusive igneous rock. It takes its name from its texture of large grains
(called porphyrites) set in a background of small grains.

METAMORPHIC ROCKS

Metamorphic rocks are formed when the minerals in rocks
are changed underground by heat and pressure. Contact metamorphic rocks are
produced when rocks are heated by magma rising through the crust. Rocks that
are folded or crushed by immense pressure deep in the crust are called regional
metamorphic rocks. The properties (characteristics) of a metamorphic rock
depend on its parent rock (the original rock type) and how it was formed.

Migmatite is a mixture of dark-coloured schist or gneiss and a
lighter coloured rock similar to granite. This piece was found in the Highlands
of Scotland.

Slate is fine-grained and dark grey or green. It splits easily
into flat sheets, and is used to make roof and floor tiles. It is formed from
the sedimentary rocks mudstone or shale.

In a schist, lines of crystals can often be seen with the naked
eye. This sample contains large crystals of garnet. Schists are mostly
medium-grained and come from shales or granites.

Gneiss (pronounced nice) is a coarse-grained, grey, or pink
regional metamorphic rock formed from limestone or granite. Light and dark
layers of minerals can be seen rippling across the rock.